WASHINGTON— U.S. doctors said on Wednesday they have succeeded in coaxing the regeneration of muscle tissue lost in people who suffered traumatic injuries, including wartime bomb wounds, with a new type of treatment that uses material from a pig's bladder.

Implanting the pig material at the wound site enticed the patient's own stem cells -- master cells that can transform into various kinds of cells in the body -- to become muscle cells and regenerate tissue that had been lost, the researchers said.

The study was small, involving only five male patients, but its results suggested that this procedure could offer new hope to a category of patients, including troops who suffered major war injuries, with scant good treatment options, they added.

All five patients, including two U.S. soldiers hurt by bombs planted by insurgents, had badly damaged leg muscles.

The research was backed by $3 million in funding over five years from the U.S. Defense Department, said Dr. Stephen Badylak of the University of Pittsburgh, who led the study.

Thousands of American troops have been left with serious physical impairments after sustaining wounds involving major loss of muscle tissue in roadside bombings and other incidents since 2001 in the wars in Iraq and Afghanistan. When a large amount of muscle is lost in vehicle crashes, industrial accidents, bomb blasts or other traumas, the body is unable to replace it and the site forms scar tissue that lacks the functionality of the lost muscle.

Existing treatments include surgery to remove scar tissue or replace it with muscle from somewhere else in the body, but these methods do not yield satisfying results and are hard on patients, the researchers said.

"Nothing has ever worked. There's been multiple things tried: the hype and the hope of stem cell therapy, new surgical techniques," said Badylak.

This study, published in the journal Science Translational Medicine, demonstrated for the first time the regeneration of functional muscle tissue in people with major muscle loss.

"While the number of patients was small, we were very encouraged by the data. And we were seeing very dramatic improvements in quality of life for some of our patients," added Dr. J. Peter Rubin of the University of Pittsburgh School of Medicine, another of the researchers.

The doctors implanted material from a pig's urinary bladder called "extracellular matrix" - the non-cellular component including collagen present within all tissues and organs - to serve as scaffolding for the rebuilding of lost muscle mass.

This material acted as a "homing device" to recruit existing stem cells in the body to rebuild healthy muscle tissue at the site of the injury, the researchers said.

Pig parts have been used for years in surgical procedures. Pig bladder "extracellular matrix" has been used in hernia repair and fixing chest wall defects after cancer removal. Before trying the procedure in people, the researchers said they successfully tested it in mice with muscle injuries.

Wartime injuries

To take part in the study, the five men had to have lost at least 25 percent of leg muscle volume and function at least six months earlier and then completed physical therapy for three to six months until their function and strength no longer improved.

The doctors then implanted the pig material and directed the men to resume physical therapy for up to six more months. Biopsies and scans confirmed that muscle growth had taken place.

The patients hurt by bomb blasts were a 27-year-old who lost 83 percent of his thigh muscle and had undergone 50 previous surgical operations, and a 28-year-old who lost 68 percent of his thigh muscle and had 14 previous operations.

The other three men had calf injuries, including one who also came from the military but was hurt while exercising and not in combat, and two civilians with severe skiing injuries.

"Frankly, most of these patients have been through hell. These are serious injuries," Badylak said. "In fact, one or two of the patients even considered amputation at one point because they've just been through so much."

Three of them, including both soldiers hurt by bombs, were measured six months after the implantation operation as stronger in five categories by at least 20 percent - and often by far more than that. The other two men also showed broad improvement but not in all five measures, the researchers said.

Badylak said four additional patients, including one woman, have since undergone the procedure with good results.

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